I. Field of the Invention
This invention relates generally to the field of mobile wireless communications receivers, and more particularly, to a wireless communications receiver capable of operation in multiple frequency bands.
II. Background of the Invention
In recent years, multiple wireless communications standards have been deployed governing the wireless interface between base stations and the mobile handsets serviced by the base stations. A geographical area is divided into a plurality of cells, each corresponding to a base station, and with the mobile handsets in a cell serviced by the base station corresponding to the cell. The base stations in turn are linked to a base station controller which in turn are linked to the Public Switched Telephone Network (PSTN) through one or more E1/T1 lines.
The GSM (Global System for Mobile Communications) is a standard based on TDMA (Time Division Multiple Access) technology that governs wireless communications over much of Europe. The DCS (Digital Cellular System) is a TDMA-based standard that governs at least some of the wireless communications in Great Britain, and other parts of Europe. The PCS (Personal Communications System) is a TDMA-based standard that is being deployed in North America.
Each of these standards define narrow band systems which apply over different passbands. For example, the receive band for the mobile side of the extended GSM (E-GSM) standard is 925-960 MHz; that for the DCS standard is 1805-1880 MHz respectively; and that for a version of the PCS standard known as PCS 1900 is 1930-1990 MHz. The channel bandwidth for the E-GSM, GSM, PCS, and DCS standards is 200 kHz. As is known, TDMA is a technology in which each frame of information transmitted over the wireless interface is divided into a predetermined number of time slots, wherein a caller is assigned one of these slots for the duration of the call. Other wide band standards have been deployed, such as the IS-95A standard, which are based on CDMA (Code Division Multiple Access) technology. Additional detail about the GSM standard is available from the European Digital Cellular Telecommunications Systems GSM Recommendation 05.05, which is hereby fully incorporated by reference herein as though set forth in full. Additional detail about the PCS1900 standard is available from T1P1.5, the appropriate U.S. based standards committee.
Each of the foregoing three TDMA-based standardsxe2x80x94E-GSM, DCS, and PCSxe2x80x94largely governs distinct geographical areas around the world. Moreover, no one of these three standards has yet achieved universal worldwide coverage. On the other hand, users are demanding mobile universal coverage from a single handset, and are understandably resistant to the need to carry around multiple handsets to deal with the different standards that are likely to be encountered.
Dual mode handsets are configured to handle two of the standards likely to be encountered. However, unique problems exist which have heretofore prevented extension of this development to a handset capable of handling three or more bands. One such problem is the excessive loading caused on the receiver components for a selected band by the receiver components for the other two bands. Another problem is the excessive space consumed by the receiver components for the three different bands. This latter problem is particularly acute given the demands of users for compact and easily portable handsets.
Therefore, there is a need for a compact, mobile wireless communications handset and/or receiver which is capable of handling wireless communications in accordance with any of the foregoing three TDMA-based wireless communications standards.
In accordance with the purpose of the invention as broadly described herein, there is provided a filter system comprising three or more filters each having a port, and each having a passband, wherein a first impedance adjusting network is coupled to the ports of each of at least two of the filters and at least one filter system port. The impedance adjusting network is configured to adjust the port impedances of each of at least two of the filters coupled to the network such that each of the adjusted port impedance of each the at least two filters coupled to the network, at a frequency representative of the passband of at least one of the other filters coupled to the network, is at a non-loading level.
In one embodiment, the first impedance adjusting network is coupled between at least one filter system input and the inputs of at least two of the filters. In this embodiment, the impedance adjusting network is configured to adjust the input impedances of each of the at least two filters coupled to the network to be at a non-loading level at a frequency representative of the passband of at least one of the other filters coupled to the network. In a second embodiment, the first impedance adjusting network is coupled between the outputs of at least two of the filters and at least one filter system output. In this embodiment, the impedance adjusting network is configured to adjust the output impedances of each of the at least two filters coupled to the network to be at a non-loading level at a frequency representative of the passband of at least one of the other filters coupled to the network.
In a third embodiment, which is dependent on the first embodiment, a second impedance adjusting network is coupled to the outputs of each of at least two of the filters, wherein the first network is configured so that the adjusted input impedance of each filter at a frequency within the passband of at least one of the other filters is at a non-loading level, and wherein the second network is configured so that the output impedance of each filter coupled to the second network at a frequency within the passband of the other filter is at a non-loading level.
In one implementation, a non-loading level of an impedance is defined as infinite impedance as indicated on a Smith chart. In one embodiment, the filter system is an element of a wireless communications receiver and/or handset.
In one implementation, the first impedance adjusting network comprises third and fourth impedance adjusting networks. The third impedance adjusting network is coupled to the inputs of two of the filters. The third impedance adjusting network is configured to adjust the input impedance of each of the two filters at a frequency representative of the passband of the other of the two filters, and vice-versa, so that the adjusted input impedance appears infinite. The third impedance adjusting network has an input. The fourth impedance adjusting network is coupled to the output of the third network and the input of a third filter. The fourth network is configured to adjust the input impedance of the third network at a frequency representative of the passband of the third filter, so that the adjusted input impedance appears infinite, and to adjust the input impedance of the third filter at a frequency representative of the passbands of the first and second filters, again, so that the adjusted input impedance appears infinite.
In one implementation example, the first filter is a filter having a passband generally coincident with the DCS band, the second filter is a filter having a passband generally coincident with the PCS band, and the third filter is a filter having a passband generally coincident with the E-GSM or GSM bands.
In one implementation, a first transmission line is coupled at one end to the input of the first filter, a second transmission line is coupled at one end to the input of the second filter, and the other ends of the two transmission lines are coupled together to form a first node. The length of the first line is such that the adjusted input impedance of the first filter, at the center frequency of the passband of the second filter, is indicated on a Smith chart as being infinite. The length of the second line is such that the adjusted input impedance of the second filter, at the center frequency of the passband of the first filter, is indicated on a Smith chart as being infinite.
A third transmission line is coupled at one end to the output of the first filter, and a fourth transmission line is coupled at one end to the output of the second filter. The other ends of the third and fourth transmission lines are coupled together to form a second node. The length of the third line is such that the adjusted output impedance of the first filter, at the center frequency of the passband of the second filter, is indicated on a Smith chart as being infinite. The length of the fourth line is such that the output impedance of the second filter, at the center frequency of the passband of the second filter, is indicated on a Smith chart as being infinite.
A fifth transmission line is coupled at one end to the first node, and a sixth transmission line is coupled at one end to the input of the third filter. The other ends of the fifth and sixth transmission lines are coupled together to form a third node. The length of the fifth line is such that the adjusted input impedance at the first node, at the center frequency of the passband of the first filter, is indicated on a Smith chart as being infinite. The length of the sixth line is such that the input impedance of the third filter, at a frequency representative of the passbands of the first and second filters, is indicated on a Smith chart as being infinite.
In one implementation example, the required lengths of transmission lines are determined using the formula:   Δ  ⁢      xe2x80x83    ⁢  X  xc3x97            c      xc3x97      K                      f        c            xc3x97              v        p            
, where xcex94X is the required length of the transmission line in terms of xcex (wavelength), c is the speed of light in meters per second (3xc3x97108 m/sec.), K is a conversion constant for converting to a desired unit of physical length, fc is the center frequency of the passband of another one of the filters, and vp is the velocity of propagation through the material making up the line. According to this implementation example, xcex94X is determined as the length of line required to move the location of the reflection coefficient measured at the filter port to a point indicative of infinite impedance on a Smith chart.
In a fourth embodiment, a filter system is provided comprising n filters, wherein n is an integer equal to three or more, each of the filters having a passband, a port, and a port impedance. A first impedance adjusting network is coupled to the ports of each of at least two of the filters, and at least one filter system port. The first impedance adjusting network is configured to adjust the port impedance of the at least two filters at a frequency characteristic or representative of at least one of the other filters coupled to the network, such that the adjusted port impedance is at a non-loading level. In a first implementation, the first impedance adjusting network is coupled between at least one filter system input, and the inputs of at least two of the filters, and is configured to adjust the input impedance of each of the at least two filters coupled to the network at a frequency representative of the passband of at least one of the other filters coupled to the network such that the adjusted input impedance of each of the at least two filters is at a non-loading level. In a second implementation, the first impedance adjusting network is coupled between the outputs of at least two of the filters, and at least one filter system output, and is configured to adjust the output impedance of each of the at least two filters coupled to the network at a frequency representative of the passband of at least one of the other filters coupled to the network such that the adjusted output impedance of each of the at least two filters is at a non-loading level.
In a fifth embodiment, dependent on the foregoing first implementation, a second impedance adjusting network is coupled between the outputs of at least two of the filters and at least one filter system output, wherein the second impedance adjusting network is configured to adjust to a non-loading level the output impedance of each of the at least two filters coupled to the network at a frequency characteristic or representative of the passband of at least one of the other filters coupled to the network.
A method of operation comprises the steps of receiving an RF signal, passing the signal through one of n filters, wherein n is an integer equal to three or more, the one filter having a passband corresponding to the frequency of the signal, while substantially blocking passage of the signal through each of the other filters, and outputting the filtered signal.
A method of implementation comprises the steps of providing n filters, wherein n is an integer equal to three or more, adjusting a port impedance of at least one of the filters so that the adjusted port impedance of each of the at least one filters at a frequency within the passband of at least one of the other filters is at a non-loading level, and performing the foregoing steps for at least one of the other filters.